The HI4005-40 fill solution is a chloride ion-containing electrolyte that is required to provide a reference potential for the HI4105 carbon dioxide gas sensing ion selective electrode (ISE). As carbon dioxide diffuses through the gas permeable membrane, the thin film of electrolyte on the surface of the pH sensing glass undergoes a pH change. The change in pH of the filling solution is proportional to the concentration of carbon dioxide gas in the solution. The HI4005-40 is four bottles of 30 mL carbon dioxide filling solution that ensure optimum measurement performance from the HI4105 carbon dioxide ISE.